Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
  • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
  • C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
  • F16L9/00—Rigid pipes
  • F16L9/02—Rigid pipes of metal
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals

Definitions

• the present invention relates to a heat-resistant austenitic stainless steel suitably used as a heat transfer tube material for a boiler, and a stainless steel pipe obtained from such a heat-resistant austenitic stainless steel, and particularly, a heat-resistant austenitic stainless steel excellent in scale peeling resistance. It relates to steel and stainless steel pipes.
• the oxidation resistance is generally further improved by shot peening the inner surface of the steel pipe. Although such a shot peening process causes an increase in cost, it is necessary to perform the shot peening process in order to ensure sufficient oxidation resistance from the viewpoint of long-term reliability of 10 years or more.
• Patent Document 1 discloses a technique for improving the steam oxidation resistance by performing a particle spray peening process on a steel material to which a rare earth element (REM) is added.
• Patent Document 2 proposes a technique for suppressing peeling of the oxide scale while improving the steam oxidation resistance by setting the surface roughness after the shot peening treatment to a certain level or more.
• Patent Document 3 proposes a technique for improving high-temperature steam oxidation resistance by performing shot peening treatment with a Cr concentration of a steel material at a certain level or higher.
• Patent Documents 1 and 3 basically suppress the growth rate of the scale by shot peening, so that the effect of suppressing the separation of the oxide scale associated with the operation / stop of the power generation facility is sufficient. It is not always possible to obtain it.
• the technique of patent document 2 although there exists an effect which suppresses peeling of an oxide scale by controlling the surface roughness of a shot peening process surface, the roughness of the initial steel material surface by the oxide scale peeling is There is a problem that the effect cannot be maintained against the oxide scale peeling which is lost and repeated, and sufficient characteristics cannot be maintained for a long time.
• the prior art for suppressing the peeling of the oxide scale includes a technique for preventing the oxide scale itself from being formed (Patent Documents 1 and 3) and a technique for preventing the generated oxide scale from being peeled (Patent Document 2).
• Patent Documents 1 and 3 a technique for preventing the generated oxide scale from being peeled
• Patent Document 2 the technology that does not form the oxide scale itself only reduces the growth rate of the scale, and there is no guarantee that the oxide scale will not be formed over a long period of several decades. Is required. For this reason, a technique for preventing the generated oxide scale from being peeled off is necessary, but the related art in this respect is not actually capable of exerting a sustained effect against repeated peeling.
• the present invention has been made under such circumstances, and the object thereof is applied to a steel pipe whose inner surface is processed by shot peening, etc., among heat transfer tubes used in a thermal power generation facility.
• An object of the present invention is to provide an austenitic stainless steel with improved properties and a stainless steel pipe made of such stainless steel.
• the heat-resistant austenitic stainless steel of the present invention that has solved the above problems is C: 0.02 to 02% (meaning mass%, hereinafter the same for chemical composition), Si: 0.1 to 1.5%, Mn: 0.1 to 3%, Ni: 7 to 13%, Cr: 16 to 20%, Cu: 0.4 to 4%, Nb: 0.05 to 0.6%, Ti: 0.05 to 0 .6%, Zr: 0.05 to 0.35%, Ce: 0.005 to 0.1%, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S: 0.005% or less (not including 0%) and P: 0.05% or less (not including 0%), respectively, the balance is made of iron and inevitable impurities, and the depth from the surface to the depth of 50 ⁇ m
• the ratio (Hv 1 / Hv 0 ) of the average hardness (Hv 1 ) and the average hardness (Hv 0 ) in the center in the thickness direction is 1.20 or more It has a gist in
• the heat-resistant austenitic stainless steel of the present invention may further include (a) Co: 3% or less (not including 0%), Mo: 3% or less (not including 0%), and W: 5% or less as required. (B) Ca: not more than 0.005% (not including 0%) and / or Mg: not more than 0.005% (not including 0%) ), (C) V: 0.6% or less (not including 0%), Ta: 0.6% or less (not including 0%), and Hf: 0.6% or less (not including 0%) It is also useful to contain one or more selected from the group, and the characteristics of the heat-resistant austenitic stainless steel are further improved depending on the components contained.
• Stainless steel pipes manufactured from heat-resistant austenitic stainless steel as described above are extremely useful as heat transfer pipes for thermal power generation facilities.
• the heat-resistant austenitic stainless steel of the present invention is less susceptible to oxide scale peeling even after repeated temperature changes associated with the operation / stopping of thermal power generation facilities, and suppresses internal oxide scale scattering when used as a heat transfer tube. It is possible to reduce the blockage of the heat transfer tube and damage to the turbine.
• the present inventors have developed the steel surface whose surface hardness has been increased by treatment such as shot peening and the like. The relationship between chemical composition was examined from various angles. As a result, the austenitic stainless steel having the same chemical composition as that of the 18Cr-8Ni austenitic stainless steel containing Ni and Cr is contained in a predetermined amount of Zr and Ce, and shot peening is performed on the steel surface. As a result of the treatment, the inventors have found that the remarkably excellent scale peel resistance can be exhibited, and the present invention has been completed.
• the heat-resistant austenitic stainless steel of the present invention is characterized in that it contains a predetermined amount of Zr and Ce, and the hardness in the vicinity of the surface is made constant by shot peening treatment or the like.
• the reason for setting the Ce content and surface hardness range is as follows.
• the preferable lower limit of the Zr content is 0.10% or more (more preferably 0.15% or more), and the preferable upper limit is 0.3% or less (more preferably 0.25% or less).
• the preferable minimum of Ce content is 0.01% or more (more preferably 0.015% or more), and a preferable upper limit is 0.05% or less (more preferably 0.03% or less).
• pure Ce may be added as a raw material of Ce, but it is also possible to add the necessary pure Ce using a separately prepared Ce-containing mother alloy or Ce-containing misch metal. Even if La, Nd, Pr, etc. contained are contained in steel as impurities at a lower concentration than Ce, there is no problem, and compared with pure Ce that is easy to oxidize, it is possible to use a master alloy or misch metal at the time of melting work. Handling can be simplified.
• Patent Document 2 is the same technique for improving the scale peeling resistance as the present invention, the surface roughness is lost when the scale peels repeatedly because of the effect due to the roughness of the steel surface. Even if the layer remains, the long-term effect cannot be maintained. Since this invention prescribes
• the average hardness (Hv 1 ) in the vicinity of the surface is the average hardness of the base material (that is, the position indicating the characteristics of the base material). It is important that the average hardness at a central portion in the thickness direction: Hv 0 ) is higher than a predetermined value. From such a viewpoint, the ratio (Hv 1 / Hv 0 ) between the average hardness (Hv 1 ) near the surface and the average hardness (Hv 0 ) of the base material needs to be 1.20 or more.
• the value of the ratio (Hv 1 / Hv 0 ) is preferably 1.5 or more, more preferably 1.8 or more.
• the upper limit of the ratio value (Hv 1 / Hv 0 ) is about 2.5 in the heat-resistant austenitic stainless steel of the present invention.
• the reason why the vicinity of the surface is defined as “from the surface to a depth of 50 ⁇ m in the thickness direction” is that the hardness is increased.
• the addition of Zr and Ce and the adjustment of the surface hardness are important requirements, but the chemical composition of each element other than the above (C, Si, Mn, Ni , Cr, Cu, Nb, Ti, B, N, S, and P) need to be adjusted appropriately.
• the effects of these components and the reasons for setting the range are as follows.
• C is an element that has the effect of forming carbides in a high-temperature use environment and improving the high-temperature strength and creep strength necessary as a heat transfer tube.
• C is 0.02. % Or more must be contained.
• the preferable lower limit of the C content is 0.05% or more (more preferably 0.09% or more), and the preferable upper limit is 0.18% or less (more preferably 0.15% or less).
• Si 0.1 to 1.5%
• Si is an element having a deoxidizing action in molten steel. Even if it is contained in a very small amount, it effectively works to improve oxidation resistance. In order to exert these effects, the Si content needs to be 0.1% or more. However, when the Si content is excessive and exceeds 1.5%, the toughness is reduced.
• the preferable lower limit of the Si content is 0.2% or more (more preferably 0.3% or more), and the preferable upper limit is 0.9% or less (more preferably 0.8% or less).
• Mn 0.1 to 3%
• Mn is an element having a deoxidizing action in molten steel, and also has an action of stabilizing austenite.
• the Mn content needs to be 0.1% or more. However, if the Mn content is excessive and exceeds 3%, hot workability is impaired.
• the preferable lower limit of the Mn content is 0.2% or more (more preferably 0.3% or more), and the preferable upper limit is 2.0% or less (more preferably 1.8% or less).
• Ni has an effect of stabilizing austenite, and it is necessary to contain 7% or more in order to maintain the austenite phase. However, if the Ni content becomes excessive and exceeds 13%, the cost will increase.
• the preferable lower limit of the Ni content is 8.0% or more (more preferably 9.0% or more), and the preferable upper limit is 12.0% or less (more preferably 11.0% or less).
• Cr 16-20%
• Cr is an essential element in order to develop corrosion resistance as stainless steel. In order to exert such effects, it is necessary to contain 16% or more of Cr. However, if the Cr content is excessive and exceeds 20%, the stability of austenite at high temperatures is lacking, leading to a decrease in high temperature strength.
• the preferable lower limit of the Cr content is 16.5% or more (more preferably 17.0% or more), and the preferable upper limit is 19.5% or less (more preferably 19.0% or less).
• Cu is one of the main strengthening mechanisms in stainless steel that forms fine precipitates in the steel and significantly improves high temperature creep strength. In order to exert this effect, the Cu content needs to be 0.4% or more. However, even if the Cu content is excessive and exceeds 4%, the effect is saturated.
• the minimum with preferable Cu content is 1.0% or more (more preferably 1.5% or more), and a preferable upper limit is 3.7% or less (more preferably 3.5% or less).
• Nb 0.05 to 0.6%
• Nb is an element effective for improving the high-temperature strength by precipitating carbonitride (carbide, nitride, or carbonitride), and this precipitate suppresses the coarsening of crystal grains and diffuses Cr. By promoting the above, a secondary effect of improving corrosion resistance is exhibited.
• Nb needs to be contained in an amount of 0.05% or more. However, if the Nb content exceeds 0.6% and becomes excessive, the precipitates become coarse and the toughness is reduced.
• the preferable lower limit of the Nb content is 0.10% or more (more preferably 0.15% or more), and the preferable upper limit is 0.5% or less (more preferably 0.3% or less).
• Ti 0.05 to 0.6%
• Ti exhibits the same effect as Nb, but by adding it in combination with Nb and Zr, the precipitates are further stabilized and effective in maintaining high-temperature strength for a long period of time.
• the Ti content needs to be 0.05% or more.
• the preferable lower limit of the Ti content is 0.10% or more (more preferably 0.15% or more), and the preferable upper limit is 0.5% or less (more preferably 0.3% or less).
• B has the effect of promoting the formation of M 23 C 6 type carbide (M is a carbide forming element), which is one of the main strengthening mechanisms, by forming a solid solution in steel.
• M is a carbide forming element
• the B content needs to be 0.0005% or more.
• a preferable lower limit of the B content is 0.001% or more (more preferably 0.0012% or more), and a preferable upper limit is 0.004% or less (more preferably 0.003% or less).
• N has the effect of improving high temperature strength by solid solution strengthening by dissolving in steel, and is effective in improving high temperature strength by forming nitrides with Cr, Nb under a long period of high temperature load. It is an element. In order to exhibit these effects effectively, the N content needs to be 0.001% or more. However, if the N content becomes excessive and exceeds 0.15%, hot workability is impaired.
• a preferable lower limit of the N content is 0.002% or more (more preferably 0.003% or more), and a preferable upper limit is 0.05% or less (more preferably 0.02% or less).
• S 0.005% or less (excluding 0%)
• S is an unavoidable impurity, but when its content increases, hot workability deteriorates, so it is necessary to make it 0.005% or less. Further, S impairs the action of adding Ce by fixing Ce as a sulfide, so S is preferably suppressed to 0.002% or less (more preferably 0.001% or less).
• P 0.05% or less (excluding 0%)
• P is an inevitable impurity, but if its content increases, weldability is impaired, so it is necessary to make it 0.05% or less. Preferably it is good to suppress to 0.04% or less (more preferably 0.03% or less).
• the contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities.
• the unavoidable impurities elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc., and Ce are rare earth elements (REM). ) May be allowed to be mixed with elements such as La, Nd, and Pr other than Ce.
• REM rare earth elements
• low melting point impurities such as Sn, Pb, Sb, As, and Zn derived from scrap raw materials reduce the strength of grain boundaries during hot working or when used in a high temperature environment. In order to improve the inter-workability, a lower concentration is desirable.
• the steel material of this invention may contain the following component as needed, and the characteristic of steel materials is further improved according to the kind of element contained.
• Co, Mo, and W have the effect of improving the high temperature strength by solid solution strengthening, and can be further increased by containing them if necessary.
• Co has the effect of stabilizing austenite like Ni, but if the content exceeds 3%, it will contaminate the melting furnace as a radioactive element, so it is preferably made 3% or less. More preferably, it is 2.5% or less (more preferably 2.0% or less). If the Mo content is excessive, hot workability is hindered, so it is preferably made 3% or less.
• the W content is excessive, a coarse intermetallic compound is formed and the high temperature ductility is lowered. More preferably, it is 4.5% or less (more preferably 4.0% or less).
• the preferable lower limit for effectively exhibiting the above effects is 0.1% or more (more preferably 0.5% or more) for Co and 0.1% or more (more preferably 0.5%) for Mo. %, And W is 0.1% or more (more preferably 1.0% or more).
• the contents may be set according to the required amount of reinforcement and the allowable cost.
• Ca and Mg react with sulfur in molten steel to form sulfides, it is possible to reduce the sulfur concentration in steel and improve the hot ductility of the steel material. If these elements are contained in an amount exceeding 0.005%, the upper limit is set to 0.005% because there are restrictions on the work such as bumping of the molten steel during the melting work. Preferably, both are 0.002% or less.
• V 0.6% or less (not including 0%), Ta: 0.6% or less (not including 0%) and Hf: 0.6% or less (not including 0%)
• V, Ta, and Hf are elements that form carbides and nitrides, and the high-temperature strength can be improved by adding to the specified components as necessary in the present invention. In any case, if it is less than 0.05%, a sufficient effect cannot be obtained, and if it exceeds 0.6%, the precipitate becomes excessive and the hot workability is impaired.
• the preferred lower limit is 0.10% or more (more preferably 0.15% or more), and the preferred upper limit is 0.5% or less (more preferably 0.3% or less).
• the crystal grain size of the base material is less than 7 in terms of ASTM (American Society for Testing and Materials) grain size number, thereby providing oxidation resistance and scale peeling resistance.
• ASTM American Society for Testing and Materials
• the particle size number is determined by ASTM, and means a particle size number calculated by a counting method (Planimetric method).
• the particle size number is more preferably 6 or less, and even more preferably 5 or less.
• the crystal grain size range as described above can be obtained by adjusting the amount of components contributing to pinning of grain boundaries and the conditions of hot and cold working and heat treatment such as drawing and extrusion during the pipe making process. .
• Each optimum condition varies depending on these three factors. For example, in order to make the crystal grain size fine, it is necessary to add a large amount of precipitated elements, to increase the degree of processing, and to lower the heat treatment temperature.
• Cold / hot working is aimed at adjusting the thickness and adjusting the grain structure by heat treatment after processing by introducing strain, and is usually carried out at a cross-section reduction rate of 30% or more.
• the heat treatment is intended to remove strain, and is generally performed in a temperature range of 1000 ° C. or higher and lower than 1300 ° C.
• a prescribed particle size range can be obtained by setting the heat treatment temperature to 1200 ° C. or higher, preferably 1250 ° C. or higher, particularly preferably 1300 ° C. or higher. It is not limited to this condition depending on the balance between processing and heat treatment.
• shot peening is desirable, and particles such as martensite steel, alumina, zirconia, and the like, which are generally referred to as projectiles (shot grains) having a diameter of several tens of ⁇ m to several mm. Is sprayed onto the workpiece at an arbitrary pressure of approximately 10 kgf / cm 2 (0.98 MPa) or less.
• surface machining such as cutting, polishing, and grinding, and shot blasting may be used, and it is important to obtain the above-described hardness ratio, and the present invention is not limited to these methods.
• the heat-resistant austenitic stainless steel of the present invention is premised on being formed into a steel pipe, and its thickness (plate thickness) is assumed to be about 5 to 20 mm.
• Example 1 A 20 kg ingot prepared by melting various steel materials (steel types A to X) having the chemical composition shown in Tables 1 and 2 below and melting in a vacuum melting furnace (VIF) is heated to a dimension of width: 120 mm ⁇ thickness: 20 mm. After hot forging and heat treatment at 1250 ° C., the thickness was reduced to 13 mm by cold rolling. Thereafter, heat treatment was again performed at 1200 ° C. for 5 minutes, and this was used as a base material.
• VIF vacuum melting furnace
• a 20 mm ⁇ 30 mm ⁇ 2 mm steel material was cut out from the base material by machining, and a test piece was prepared by smoothing and mirror-finishing the surface of the steel material by polishing using emery paper and buffing using diamond abrasive grains.
• steel types A to Q satisfy the requirements specified in the present invention (invented steel), and steel types R to X deviate from the requirements specified in the present invention ( Comparative steel).
• Steel types J and K are obtained by adding pure Ce using misch metal and containing La and Nd as impurities.
• the steel material obtained above is shot-peened using alumina particles (shot particle size: 100 ⁇ m) in four stages of spraying pressure 1, 2 , 4, 6 kgf / cm 2 , and the cross section of the dummy sample is mirror polished. Then, the ratio of the average hardness (Hv 1 ) from the surface to the depth of 50 ⁇ m in the thickness direction and the average hardness (Hv 0 ) of the central portion in the thickness direction was measured. Using these various test pieces, a repeated oxidation test was conducted to evaluate the thinning amount (mass loss amount).
• the average hardness (Hv 0 ) of the base material was measured by measuring the hardness at three locations at intervals of 1 mm in the direction perpendicular to the thickness at the center of the plate thickness and averaging them.
• the average hardness (Hv 1 ) in the vicinity of the surface was measured by measuring three locations at equal intervals from the outermost surface to 50 ⁇ m in the thickness direction of the cross section, and averaging these three data.
• test no. 69 In to 72 which reduces the mass loss of -103mg ⁇ cm -2 thickness reduction from 232 mg ⁇ cm -2 to 129 mg ⁇ cm -2, the surface hardness by independent shot peening the chemical composition It can be seen that the increase in has a certain effect on the reduction in mass loss.
• the samples of steel grades A to Q and steel grades R to X are compared between samples that are not sufficiently effective in shot peening and that are substantially the same as the samples that have not been processed (the uppermost numerical value of each steel grade). Then, even if the shot peening treatment is substantially the same as that which is not performed, it can be seen that a certain improvement effect can be obtained by setting the chemical component composition within the range defined in the present invention. For example, test no. 1 and test no. Comparing 69 and the steel grade R and changed to the chemical compositions of steels A, from 232 mg ⁇ cm -2 to 186 mg ⁇ cm -2, so that you can -46mg ⁇ cm -2 improvement.
• the heat-resistant austenitic stainless steel of the present invention is useful as a heat transfer tube material for boilers of thermal power generation facilities, and is excellent in scale peel resistance.

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